Clonal cell lines of neural or pituitary origin were assayed for the presence of peptides with opiate-like properties by determining the ability of peptides extracted from cells to inhibit adenylate cyclase of NG108-15 neuroblastoma x glioma hybrid cells. Opioid peptides were found in cells of the AtT-20 mouse pituitary tumor clonal cell line, which is known to synthesize ACTH. Preliminary characterization of these peptides suggests that they resemble alpha-, Beta-, and gamma-endorphin and Met-enkephalin, which are opioid peptides recently found in brain and the pituitary and which are thought to be synthesized by specific cleavage of the peptide Beta-lipoprotein. Ongoing studies include further characterization of the opioid peptides and investigations of the regulation of their biosynthesis and secretion. BIBLIOGRAPHIC REFERENCE: Giagnoni, Gabriella, Sabol, Steven L. and Nirenberg, Marshall: Synthesis of opiate peptides by a clonal pituitary tumor cell line. Proc. Natl. Acad. Sci. USA 74: 2259-2263, 1977.